The intrinsic value of faceted gemstones is a well recognized fact and commercial transactions in which these items change ownership may involve considerable sums of money. For this reason it is imperative that the prospective buyer have a means of ascertaining whether or not the gems in question are indeed what the seller claims them to be.
Because of their highly polished surfaces, any tests the prospective buyer may be allowed to make must be completely non-destructive, and this limitation has resulted in the development of the science of gemology. Heretofore the most important instrument of the gemologist for identification work has been the refractometer. This instrument in the hands of a trained gemologist is capable of accurately indicating the refractive index (n.sub.D) of the gem. When this value has been determined, the trained gemologist refers to reference books to determine those possible gems which have the determined value of n.sub.D. Accurate values of n.sub.D have been determined and recorded for all gemstones. Depending upon other characteristics -- e.g., color, pleochroism, or birefringence of those possibilities -- he can either make a firm identification or determine what other test he must make.
For the confirmation of the purported identity of most common gems, the n.sub.D value is usually sufficient, as those gems most similar in appearance usually have significantly different n.sub.D values.
The range of the gemologist's refractometer is greatly limited by the requirements of its optical system (which determines the critical angle but converts that value into n.sub.D which is read on the scale) and this instrument cannot be used to determine n.sub.D values above 1.81. Many important gems have higher values of n.sub.D.
About 1825 Fresnel and Snell developed equations relating the ratio of reflected light:incident light to the index of refraction (n.sub.X where X is the wavelength of incident light) of a transparent solid.
Attempts have been made to construct refractometers based on this principle (Fresnel's) as they should be capable of measuring the refractive index (n.sub.D) of all gems.
The purpose of such a refractometer would be to accurately measure n.sub.D so that this value can be used to make a judgement based upon the published reference values for the minerals. Without an accurate value of n.sub.D, erroneous conclusions will be drawn.
A refractometer of this type must use monochromatic sodium light in order to get accurate values of n.sub.D. The index of refraction (n) of a gem material varies with the wavelength of the light used for the measurement. The term used to describe this change is dispersion. Dispersion is defined as (n.sub.C -n.sub.F) where C and F represent wavelengths corresponding to the F (blue) and C (red) lines of the hydrogen spectrum. Dispersion is the property which imparts the flash of "fire" in a diamond. Over reasonably short ranges the slope of ##EQU1## is constant and one can estimate the correction required to convert n.sub.X (the value determined by use of a different light source than a sodium lamp) to the value n.sub.D which is needed to confirm an identification. ##EQU2## Since the value (n.sub.C -n.sub.F) is different for each gem and ranges from less than 0.01 to over 0.30, this correction factor becomes increasingly important as X deviates from 590 nm (D line of sodium).
Because of the dependence of the correction factor on the dispersion, it is impossible to convert an n.sub.X value to an n.sub.D value on an unknown specimen. Consequently it is impossible to construct a refractometer based upon Fresnel's equations which can give accurate n.sub.D values unless a light source of 590 nm is used. Because of the need to make very accurate measurements of n.sub.D and the subsequent interpretation of that value, it has been impossible for the unskilled individual to make accurate gem identifications. As a result, he must rely on the honesty of the seller to provide the correct identification. Because of this fact alone, great numbers of citizens each year buy gems of quartz under the mistaken impression that they are purchasing topaz. The problem is even greater in foreign countries where a visitor is offered "rare native stones at good prices". Upon returning with his purchases he is told by the gemologist that he has purchased "junk at inflated prices".
The commercial gem buyer usually has to travel to foreign lands to purchase gems near their source. In order for him to confirm the identity of the rare or unusual gems offered to him, it is necessary for him to carry his sophisticated equipment with him at a considerable inconvenience. Both the gem dealer and the untrained gem buying citizen have need for an instrument which is easy to carry, simple to operate and can accurately confirm or deny the validity of a purported identification.
This invention is based on the heretofore unrecognized concept that gem identifications can be confirmed by optical means without the determination of the refractive index. In order to perform this task, this instrument provides means for sensing an optical property which is different for each gem. This instrument therefore provides a means of relating that sensation into the name of the gem. Reflectivity is the property used in this invention. Fresnel's equation states that if a beam of light is directed onto the surface of a gem, the proportion of that beam which is reflected depends upon the index of refraction (n.sub.X) of the gem at the wavelength of light used. However, due to dispersion, n.sub.X cannot be converted to n.sub.D and n.sub.X is in itself meaningless as n.sub.X has not been determined for the gems and so it cannot be used by the gemologist. One other limiting factor relating to the construction of a refractometer using Fresnel's formula is the fact that in order to determine the ratio, both the amount of incident and reflected light must be determined. If only the reflected light is determined, then the light source must be so focused that all of the incident light strikes the gem and reflections come from no other source. This requirement will require accurate alignment and focusing of the light beam along with the associated manufacturing costs. The ability to focus the light beam will determine the surface of the gem which must be exposed to the beam. This in turn limits the utility of the instrument to measure the n.sub.X of small gems.